Sydney
9.1 Precipitation on surface water

The total volume of precipitation on surface water in the 2012–13 year is 134,391 ML. The following tables provide a summary and breakdown of precipitation on the surface water assets in the Sydney region.

Precipitation on surface water during the 2012–13 year

Surface water asset	Precipitation (ML)
Storages managed by the Sydney Catchment Authority	117,007
Storages not managed by the Sydney Catchment Authority	7,962
River sections	9,422
Total	134,391

 

Breakdown of precipitation on storages managed by the Sydney Catchment Authority during the 2012–13 year

Storage	Precipitation (ML)
Avon	9,547
Blue Mountains1	415
Cataract	7,949
Cordeaux	6,980
Fitzroy Falls	6,691
Lake Burragorang (Warrangamba Dam)	57,933
Lake Yarrunga (Tallowa Dam)	11,551
Nepean	3,568
Prospect	3,414
Wingecarribee	5,481
Woronora	3,478
Total	117,007

¹ Blue Mountains storages include Medlow Reservoir, Greaves Creek Reservoir, Upper Cascade Reservoir, Middle Cascade Reservoir, and Lower Cascade Reservoir.

Breakdown of precipitation on storages not managed by Sydney Catchment Authority during the 2012–13 year

Storage1	Precipitation (ML)
Bamarang	678
Bundanoon	225
Danjera	915
Farmers Creek No. 2	77
Flat Rock Creek	244
Lake Lyell	1,427
Lake Wallace	1,003
Medway	161
Pejar	1,131
Sooley	844
Thomsons Creek Dam	1,257
Total	7,9622

¹ Data for storages not listed in the above tables are not available. Precipitation for those reservoirs has been considered within line item 9.4 Runoff to surface water.

² Static surface area is used.

 

Breakdown of precipitation on rivers during the 2012–13 year

River1	From	To	Precipitation (ML)
Capertee	Glen Davis	Upper Colo	940
Coxs	Lithgow	Kelpie Point	1,482
Macdonald	Howes Valley	St Albans	134
Shoalhaven	Kadoona	Fossickers Flat	3,041
Wollondilly	Pomeroy	Golden Valley	3,825
Total			9,4222

¹ Only the river sections for which river volumes have been calculated are included. Precipitation for all the other river reaches has been considered with line item 9.4 Runoff to surface water.

² Static surface area is used.

Bureau of Meteorology (the Bureau): National Climate Centre daily climate grids (rainfall), Australian Hydrological Geospatial Fabric waterbody feature class, Australian Water Resources Information System.

The Bureau.

The Bureau provided precipitation data for the storages not managed by the Sydney Catchment Authority and for precipitation on the river sections.

Monthly precipitation data were produced by the Bureau. These were based on daily data from approximately 6,500 rain gauge stations and interpolated to a 0.05 degree (approximately 5 km) national grid (Jones et al. 2007). The average monthly surface area was calculated from daily storage levels and capacity tables. Where this data was not available a static surface area was produced from the AHGF. The surface area of the rivers was estimated using cross-section and long term average river level data and remains static in these calculations.

The precipitation at each waterbody was estimated from the proportionally weighted average of grid-points that intersected each water feature. The volume was then estimated using the surface area of each waterbody. The surface area varied dynamically with changing storage level for storages where the relationship between storage level and surface area had been derived.

• The precipitation estimates were subject to approximations associated with interpolating observation point data to a national grid detailed in Jones et al. (2007).
• The dynamic storage surface areas calculated from the levels and capacity tables represent a monthly average and therefore do not capture changes that occur on a shorter temporal scale.
• The use of the static default AHGF surface area is an approximation only. It represents the storages at capacity and therefore likely results in an overestimation of precipitation on the storages.
  

The uncertainty estimate was not quantified.

Daily return systems (DRS) database. Some stations from which DRS database receives data are managed by the Bureau. Data is also held in the Hydstra system.

Sydney Catchment Authority.

Precipitation for the following storages are managed by Sydney Catchment Authority:

• Cataract Reservoir
• Cordeaux Reservoir
• Avon Reservoir
• Nepean Reservoir
• Woronora Reservoir
• Lake Burragorang (Warragamba Dam)
• Prospect Reservoir
• Wingecarribee Reservoir
• Fitzroy Falls Reservoir
• Lake Yarrunga (Tallowa Dam)
• Blue Mountains reservoirs

Rainfall received over the entire catchment area was calculated by interpolating rainfall values from point gauges (primarily tipping-bucket rain gauges). Rainfall volumes over storages were calculated by multiplying relevant surface areas and interpolated rainfall values.

Catchment average rain assumed to fall on the surface of a lake.

The precipitation volume is based on measured data. Estimated uncertainty based on accuracy of rain gauges, limitations in the calculation method and organisational practice is +/– 10%.